In the context of the given work a new combustion concept focussing on how to burn solid biomass fuels with low emissions is developed and tested. The combustion concept distinguishes itself from the current state of scientific and technical knowledge for small scale firing plants in the following points:
- Combination of air staging and fuel staging via spatial separation of the fuel degassing, the complete combustion of the gas/air-mixture and the oxidation of the degassing residues (primary coke),
- Production of a reduction gas via transformation of primary coke and application of this reduction gas as a stage fuel (no application of a separate secondary fuel necessary) and
- Application of the blue flame burner concept on a burner firing system which burns solid biomass fuels, i.e. changeover from a diffusion flame to a premixed flame.
A burner firing system is built and tested to evaluate the developed combustion concept. According to this concept the combustion air is fed in three stages: in the area of the fuel degassing, the oxidation of the primary coke and the combustion of the gas/air-mixture. Fuel pellets made of residues from straw and grain as well as wood are used.
A strong dependence on both, the total air ratio and the partial air ratio is determined for the NOx-concentration. The best results were achieved with very little excess air. It was possible to reduce the NOx-concentration by more than 40% by reducing the total air-fuel ratio from 1.6 to 1.1. Up to a total air-fuel ratio of 1.2, stable low CO-emissions of approx. 40 mg/m³ stp (13 vol% O2) were determined. The particle concentration was reduced by 50% compared to the results from measurements taken at other combustion plants which used similar fuels.
A reduction of the NOx-concentration could also be achieved by adding the reduction gas. Optimal conditions provided (temperature > 700 °C at the area of reduction, low oxidation degree of primary coke), it was possible to reduce the NOx-concentration by more than 40 %.
The possibility of applying the blue flame burner concept to the test burner can generally be confirmed. The main difficulty in implementing a burner with a premixed flame lies in the high flow velocity of the pyrolysis gas air mixture inside the mixing zone. During the first few tests the flame frequently blew off. By introducing a flame holder, it was possible to reduce the frequency of flame extinction. However, the burner firing system was still very unstable during the operation with a premixed flame when operating conditions changed (e.g. removal of primary coke, pressure variation). In order to operate the burner with a premixed flame on a long-term and stable basis, a specially designed regulation for the operation with a premixed flame as well as the installation of an ignition device in case of flame extinction needs to be developed.